The refractory period in a neuron is the period of complete inexcitability between:

Correct answer: (b) Depolarisation and repolarisation

Explanation

After a nerve fibre fires, there is a short time in which it cannot fire again. During the absolute refractory period, no stimulus, however strong, can start a new impulse. It lasts from the start of depolarisation until most of repolarisation is over:

• during depolarisation, the sodium channels are already open;
• during repolarisation, they are inactivated and cannot open again until the membrane is nearly back at its resting potential.

This ensures that an impulse travels in only one direction. A relative refractory period follows, when only a stronger stimulus can start an impulse.